Accepted Manuscript

Title: The role of green extraction techniques in green analytical chemistry

Author: Sergio Armenta, Salvador Garrigues, Miguel de la Guardia

PII: S0165-9936(15)00131-4

DOI: http://dx.doi.org/doi:10.1016/j.trac.2014.12.011

Reference: TRAC 14444

To appear in: Trends in Analytical Chemistry

Please cite this article as: Sergio Armenta, Salvador Garrigues, Miguel de la Guardia, The role of

green extraction techniques in green analytical chemistry, Trends in Analytical Chemistry (2015),

http://dx.doi.org/doi:10.1016/j.trac.2014.12.011.

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1

The role of green extraction techniques in Green

Analytical Chemistry

Sergio Armenta, Salvador Garrigues, Miguel de la Guardia

Department of Analytical Chemistry, University of Valencia, Research Building “Jeroni Muñoz”, Dr.

Moliner str., 46100 – Burjassot (Valencia), Spain

HIGHLIGHTS

Green extraction methods are evaluated on the frame of Green Analytical Chemistry

Analyte extraction is identified as a critical step

Microextraction techniques are basic tools to greening sample treatment

ABSTRACT

Greening extraction techniques to improve the sensitivity and the selectivity of

analytical methods is the sustainable alternative to classical sample-preparation

procedures used in the past. In this update, we review the main strategies employed in

the scientific literature to reduce deleterious side-effects of extraction techniques. We

demonstrate that the evolution of sample-treatment procedures is focused on the

simultaneous improvement of the main analytical features of the method and its

practical aspects, including the economic case.

Keywords:

Green Analytical Chemistry

Extraction

Liquid-liquid extraction

Microextraction

Sample preparation

Selectivity

Sensitivity

Side-effect

Solid-phase extraction

Sustainability

Corresponding author. Tel.: +34 96 354 4838; Fax: +34 96 354 4845.

E-mail address: miguel.delaguardia@uv.es (M. de la Guardia)

1. The Green Wave

In a changing world, with the tremendous impact of human presence, the urgent need

for sustainability of all of our activities has accelerated the evolution from the

chemurgical paradigm to the ecological paradigm in which the environmental side

effects of our chemical activities must be seriously taken into consideration [1]. Green

Chemistry [2–4] and Green Analytical Chemistry (GAC) [5–9] evolved from the

academic sphere to the real world, so there is a tremendous research activity on

greening all aspects concerning the analysis of any kinds of sample, not only those for

environmental studies. We are absolutely convinced that GAC will be really useful in

the years ahead. The application of cheap, fast and environmentally safe procedures in

environmental, clinical and food analysis will improve the quality of life in developing

countries [10]. So, it can be seen that GAC has been the key tool to move from the

chemurgical paradigm to the ecological paradigm in analytical chemistry and to create

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sustainable tools for challenges in the increasing demand in analysis for a clever

combination of environment-friendly and cheap methodologies (see Fig. 1).

Based on the 12 principles of GAC [11], many green methods were proposed in

recent years, and scientific journals have published special issues regarding GAC

practice in research and applied laboratories, as can be observed in Table 1, so creating

a wave that modified the concepts and the practice of analysis.

In summary, GAC has been well accepted by the scientific community. However, the

change from qualitative to quantitative observation of the green character of analytical

methodologies has evolved much more slowly than the scientific production in the field.

In this sense, Life Cycle Assessment (LCA), a holistic tool encompassing all

environmental exchanges (i.e., resources, energy, emissions, and waste) occurring

during all stages of the life cycle of activities, is a useful tool, especially when applied

to products or services for which the life-cycle concept and its stages are clearly defined

[12]. An additional semi-quantitative criterion was developed by the Green Chemistry

Institute (GCI) of the American Chemical Society (ACS). The criterion was applied to

the National Environmental Methods Index (NEMI), a free Internet-searchable database

of environmental methods [13]. The profile criterion was based on four key terms

concerning reagents employed as:

(1) persistent, bioaccumulative and toxic (PBT);

(2) hazardous;

(3) corrosive; and,

(4) the amount and the nature of waste.

A similar criterion includes energy as a key point to be considered [14]. The addition

of energy as a criterion is important due to the high reliance on non-renewable resources

for production of electrical energy.

In recent years, an ecological scale was developed for the evaluation of analytical

methods based on the introduction of penalty points [15]. According to it, a 100 score

corresponds to a completely eco-friendly methodology, but subtracting penalty points of

the method due to the volume and the toxicity of reagents consumed, energy consumed,

emissions, operator hazard and waste generation. Methods are classified according to

the eco-scale as:

excellent green analysis (> 75 points);

acceptable green analysis (> 50 points); and,

inadequate green analysis (<50 points).

More recently, a new criterion was proposed to relate the penalty-point values to the

volumes of reagents consumed and wastes generated using mathematical expressions

and to associate the eco-scale value to a category class (A–G) in a so-called Green

Certificate [16].

2. Greening analytical procedures

Remote sensing and direct measurements on untreated samples are the green dream

of analysts and many strategies have been developed for the analysis of target

compounds based on the use of spectroscopy and electroanalytical signals [17] and

image processing [18]. However, in most analytical methodologies, sample treatment is

an unavoidable step and the use of a classical methodology, similar to that in Fig. 2

(sampling, sample transport and sample preparation before the acquisition of analytical

measurements) is absolutely necessary. Typical sample-treatment methods include

homogenization, filtration, centrifugation, clean-up, analyte extraction, preconcentration

and/or derivatization. On evaluating the environmental impact of methods, sample

preparation is, by far, the most challenging step regarding both the main features and the

green parameters of the methods. Sample dissolution and analyte extraction involve the

use of reagents and energy, and special care must be taken to select the procedure as

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simple as possible at room temperature, and the least hazardous reagents. In this

context, options for greening methods must be based on avoidance of the use of toxic

reagents and a strong reductions in consumption of energy and reagents, waste

generation, time taken and operator effort. As a result, minimization and automation

have been the basic tools for greening the analytical methods.

3. Facing the problem of sample treatment

Sample treatment has been the focus of intensive research from the GAC perspective

in the past 20 years, since it is the bottleneck of analytical procedures.

It is worth stressing that the sample-preparation step largely determines the quality of

the results obtained and is the main source of systematic errors and random lack of

precision of analytical methodologies. The sample-treatment step must guarantee a

quantitative recovery of target analytes, avoiding contamination and providing matrix

isolation as far as possible, in order to reduce potential interferences and matrix effects

during the measurement step.

We should notice that there is no universal sample-preparation technique suitable for

all types of sample, and that sample preparation depends on the matrix, the nature of

analytes and the final measurement mode. Moreover, an appropriate method for a target

analyte may not be good for comprehensive screening of compounds.

In recent years, sample-preparation methodologies evolved from hard strategies to

soft methods based on room-temperature, ultrasound-assisted leaching [19–21] or

microwave-assisted digestion using closed systems [22–24], so providing a fast, safe

methodology, especially for sample digestion and sample dissolution.

Analyte extraction has the double purpose of matrix isolation and analyte

preconcentration, and the appropriate selection of solvents and reagents and the control

of the preconcentration process are absolutely necessary in order to:

separate quantitatively the target analyte from the matrix; and,

increase the concentration level of the target analyte in the final solution to be

measured.

Sometimes, solid samples are difficult to analyze due to the need to transfer the

target analytes to a liquid phase. Leaching the analyte (i.e., solid–liquid extraction or

lixiviation) is one of the easiest, most widely used sample treatments. Classically,

leaching has been widely carried out by maceration, based on the correct choice of

solvents and the use of room temperature or controlled temperature and/or agitation to

increase the solubility of compounds and the rate of mass transfer. In general, heating

the system increases the solubilization power of the reagents or solvents used, but

involves environmental side-effects (i.e., energy consumption). Despite the extensive

use of leaching, it is characterized by long extraction protocols with low efficiency.

In 1879, Franz von Soxhlet developed Soxhlet extraction, which is the most widely

used leaching technique [25]. Soxhlet extraction is a primary reference against which

performance in new leaching methods is measured. It is still an attractive option for

routine analysis because of its general robustness and relatively low cost. The Soxhlet

system is simple and easy to use, and it enables the use of a large amount of sample

(i.e., 1–100 g). However, the main drawbacks are long extraction times and large

amounts of solvent required, which also mean that the solvent must be evaporated to

concentrate the analytes before their determination [26].

However, when samples are water or aqueous solutions with a complex matrix (e.g.,

wastewater or seawater, body fluids or juices) it can be necessary to move from the

original solution to a new phase using immiscible solvents or solid phases, suitable to

extract the analytes selectively.

In short, there are several strategies proposed in the literature for analyte extraction

[27,28], involving liquid-liquid extraction (LLE) [29-31] and solid-phase extraction

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(SPE) [32,33]. LLE and SPE are the most widely used techniques for the extraction of

liquid samples. In the first type, an appropriate selection of the extraction solvent

permits removal of the analyte from the original solution to the new phase, directly or

after a previous derivatization.

From our point of view, SPE is probably the best option to improve analyte

concentration and to separate it from a complex matrix. Generally, SPE consists of four

steps:

column conditioning;

sample loading, which implies analyte retention into the solid phase;

column post-wash; and,

analyte elution from the solid phase using an appropriate solvent.

The most common design applied in SPE is the polypropylene cartridge with placed

sorption phase, which vary in size from micro-sized disks in 1 mL syringes to 6 mL

syringes. SPE can manage relatively high volumes of samples for analyte

preconcentration being suitable to be eluted on-line with microliters of an appropriate

solvent to do their determination.

The sample preparation step can be performed off-line, at-line or on-line. At-line

procedures are performed with a robotic system or autosampler and no manual

preparation is required, which is the case of off-line systems. On the other hand, on-line

procedures combine directly the sample preparation step with the measurement mode,

usually via a multiport valve.

4. Green extraction solutions

Fig. 3 shows, as a scheme, the different variables to be considered on greening the

extraction steps which, in short, involve the nature and amount of used reagents and the

reduction of the energy employed for extraction.

As it has been aforementioned, leaching of the analyte from a solid sample is one of

the easiest and most widely used sample treatments. As a consequence, a variety of

sample preparation methods have been developed over the past decades with the

objective to improve the extraction performance as well as to reduce overall analysis

time and cost. Recent developments based on ultrasound assisted treatments [34,35]

enhance the solid-liquid equilibrium, reducing the extraction time. Ultrasonic energy

causes an effect known as cavitation, which generates numerous tiny bubbles in liquid

media and mechanical erosion of solids, including particle rupture. Sonication provides

an efficient contact between the solid and the extractant, usually resulting in a good

recovery of the analyte [36].

On the other hand, there are no doubts on the need of a quantitative evaluation of the

energy requirements of extraction steps and it is one reason to look for greener

alternatives to traditional Soxhlet [37]. Pressurized liquid extraction (PLE), also named

accelerated solvent extraction (ASE) [38-40] and microwave-assisted extraction (MAE)

are techniques that can be used instead of Soxhlet for the extraction of organic

compounds, providing a clear improvement of the extraction processes based on a

drastic reduction of time and temperature and solvent requirements.

Moreover, different alternatives combining the different strategies previously

commented have been developed to overcome the main drawbacks of the Soxhlet

extraction. For instance, high-pressure Soxhlet extraction in which the extractants do

not reach supercritical conditions and the time required and the solvents consumed are

drastically reduced [41]. The combination of Soxhlet and ultrasound assisted extraction

has been developed to take advantage of both methodologies and reduce the number of

Soxhlet cycles, greening the methodology [41].

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Between the attempts to improve Soxhlet performance, the most successful has been

the use of microwaves, being the microwave-assisted Soxhlet extraction the most

interesting improvement of conventional Soxhlet extraction [41]. The main limitations

overcame by this approach is the long extraction time periods, the possibility to

automate the procedure and the ability to quantitatively extract strongly retained

analytes.

On the other hand, the LLE procedure is suitable to be miniaturized as for example

based on the single drop extraction strategies [42-45], dispersive liquid–liquid

microextraction (DLLME) [46] and hollow fiber liquid-phase microextraction (HF-

LPME) [47]. These methods differ in design, but they all have one common feature:

namely, they use only microvolumes of organic solvent and thus comply with the

requirements of GAC. Other reported a LLME technique, is the continuous flow

microextraction (CFME). In this method, the extraction solvent drop is injected into a

glass chamber by a conventional microsyringe and held at the outlet tip of a PTFE

connecting tube, the sample solution flows right through the tube and the extraction

glass unit to waste, the solvent drop interacts continuously with the sample solution and

extraction proceeds simultaneously [48]. In recent years, it has been developed the

directly-suspended droplet microextraction technique (DSDME) [49] in which a small

volume of an immiscible organic solvent is added to the surface of the aqueous solution

gently stirred. The vortex results in the formation of a single droplet at or near the center

of rotation.

Additionally, the use of membrane-mediated [50] extraction techniques can favor

analyte extraction processes and move it from the original sample to an accepting

solution ready to be employed for analyte measurement. As indicated in Table 2, it

involves a simultaneous two-step process and a medium preconcentration, when used

on a closed circuit of the accepting phase which can be relatively easy automatized

based on flow-injection analysis (FIA) [51], sequential injection analysis (SIA) [52] or

the use of automated syringe systems [53].

Interesting alternatives to the use of classical organic solvents as extraction media

have been provided based on ionic liquids [54-57], agro-solvents, like alcohols or

terpenes [58] or the use of surfactant solutions [59,60]. The aforementioned procedures

provides specific solutions for greening classical extraction methods based on the use of

alkanes, aromatic hydrocarbons or chlorinated solvents. However, the deleterious

effects of those alternative solvents are not well known or understood, especially in the

case of ionic liquids, and it must be taken into account in order to clearly identify the

strengths, weakness, opportunities and threats of the alternative extraction process.

Recent advances in SPE extraction involve the evolution of formats, sorbent types

and modes of interaction [61]. Typically, a commonly utilized format is a

polypropylene cartridges consisting of a 20 μm frit, made of polyethylene or

polytetrafluoroethylene, at the bottom of the syringe with the relevant sorbent with an

additional frit at the top. Alternatives to SPE cartridges include disks, the SPE pipette

tip, 96-well SPE microtiter plates and also small columns, which can be on-line

connected to a liquid chromatography (LC) system. From a green perspective, the on-

line SPE is preferred. Usually, when it is coupled to a liquid chromatograph, it consists

of a small pre-column placed in a six-port high-pressure switching valve. During

injection, the sample is pre-concentrated on a pre-column and later the analytes are

eluted onto the analytical column by valve switches. The main advantages are higher

throughput and limited manual processing, as well as low cost.

An alternative to SPE is the quick, easy, cheap, effective, rugged and safe

(QuEChERS) extraction method [62], which involves the extraction of analytes from a

homogenized sample using an acetonitrile and salt solution and the clean-up of the

supernatant using a dispersive SPE (dSPE) technique. This QuEChERS approach offers

a user-friendly alternative to traditional LLE and SPE.

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The idea of scaling down SPE has led to the development of analytical

microextraction procedures. Those methods can be defined as non-exhaustive sample-

preparation steps using a very small volume (microliter range or smaller) of extracting

phase (solid, semi-solid polymeric or liquid material), relative to the sample volume.

The field of microextraction gained in significance with the invention of solid-phase

microextraction (SPME) in 1990 [63], which later, in 1993, became commercially

available. In this technique, a small amount of extracting phase dispersed on a solid

support, normally, a fused-silica fiber or a metal core, is exposed to the sample, or its

headspace, for a well-defined period of time.

The several implementations of SPME include mainly open-bed extraction concepts,

such as agitation mechanism [i.e., stir-bar sorptive extraction (SBSE), stir-rod sorptive

extraction (SRSE), stir-cake sorptive extraction (SCSE), rotating-disk sorptive

extraction (RDSE)], dispersed particles into the solution, also called dispersive SPME

(DSPME) and needles. In the last case, in-needle SPME, solid-phase dynamic

extraction (SPDE), microextraction by packed sorbent (MEPS), microextraction in a

packed syringe, fiber-packed-needle microextraction (FNME) have been proposed [64].

Novel designs for the SPME include membrane SPME (M-SPME) [65], which

involved physical separation between the polar extraction medium and the analyzed

sample by means of a membrane. Another membrane-based procedure is polymer-

coated hollow-fiber membrane (PC-HFM), a simple and inexpensive extraction

technique that involves coupling HFM with SPME and SBSE technology [66]. Another

extraction method, membrane extraction with sorbent interface (MESI) [67], consists of

a permeable silicone membrane coupled with an adsorbent trap for sampling and

concentration of organic compounds.

The use of electrochemically-aided SPME in analytical practice has also been

reported [68]. However, this particular technique has very low extraction efficiency and

cannot be coupled to a chromatographic system, so a new variant, electrosorption-

enhanced SPME (EE-SPME) [69], was proposed in 2007. Advances in

electrochemically-assisted solid-based extraction techniques were recently reviewed

[70].

Special attention must be paid to developments in solvent-free extraction methods for

sample preparation and analyte separation [71,72]. The use of thermal desorption

systems is a good option for the elution of analytes retained on solid phases and,

because of that, headspace-based methodologies have been developed for SPME

analysis of volatile and semi-volatile compounds by gas chromatography (GC) [73–75].

Those procedures can be considered a serious alternative to the use of solvents in

extraction processes and they should be seriously evaluated in order to quantify the

amount of energy consumed and, thus, their environment-friendly character.

In any case, it is clear that the replacement of classical extraction procedures with

microextraction techniques does not imply only a change of scale. It is a new concept in

which the amount of sample and reagents used substantially decreases and the speed

and sustainability of methods are improved. The main trouble is that reduction in the

sample size can affect the representativeness of analytical data, especially in the case of

the analysis of highly heterogeneous samples.

5. Future trends in green extraction

Pioneering efforts in the automation of extraction procedures through FIA have been

demonstrated to be one of the best ways to reduce operator risks and to avoid

environmental side-effects by reducing consumption of reagents and generation of

waste. However, the microscale of FIA procedures was not enough to assure their

sustainability. Additional efforts in recent years on miniaturization of extraction also

offered an interesting way to improve the greenness of analytical procedures. Recent

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developments on microfluidic systems [76] and on-chip µSPE include the use of

disposable sorbents using mesofluidic platforms [77], which open new possibilities to

green analytical methodologies. So, it is clear that in the years ahead much more effort

will be made to link these approaches, which can also be improved by clever selection

of the phases, solvents and solid, employed in the preconcentration steps to guarantee

the inert character of residues and to consume as little energy as possible.

One of the key factors in evaluating the sustainability of the different methodologies

proposed concerns the correct evaluation of environmental risks regarding reagents,

wastes and the energy employed. Those factors must be balanced in order to provide

simultaneously the best analytical features and the smallest environmental side-effects.

As indicated previously [78], the greenest methodology is that suitable to solve an

analytical problem with the minimum environmental impact, but stress must be placed

on the first part of this sentence – finding the solution to the problem.

Concerning the reagents used in the extraction steps, efforts should also be made in

evaluating new solvents (e.g., agro-solvents, ionic liquids or surfactant solutions), other

organized media {e.g., crown [79] and crypta ethers, and calixarenes) and lipidic

structures (e.g., liposomes and vesicles). The development of new solid phases suitable

for use in the selective extraction of target analytes {e.g., imprinting solid phases [80]

and nano-materials [81–83]} will contribute to improving the analytical features of the

methods and to reducing dramatically the amounts of reagents and energy used. So, we

must be optimistic about the future, and, once again, it will be demonstrated that GAC

can be a driving force to expand basic research in analytical chemistry, to make an

ethical commitment to the environment and to reduce the cost of the analytical methods,

thereby contributing to the sustainability of laboratories and enterprises.

Acknowledgements

Authors acknowledge the financial support the Ministerio de Economia y

Competitividad-Feder (Project CTQ2011-25743 and Project CTQ2012-38635) and

Generalitat Valenciana (Project PROMETEO-II 2014-077).

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Captions Fig. 1. Green Analytical Chemistry: shift from chemurgical paradigm to ecological paradigm.

Fig. 2. Strategies for greening an analytical method.

Fig. 3. Aspects to be considered on greening extraction procedures.

Table 1. Special issues of journals on Green Analytical Chemistry

Journal Special issue (no. of papers) Year, vol. (no.)

The Analyst (RSC) Environmentally Conscientious Analytical

Chemistry (5)

1995, 120 (2)

Spectroscopy Letters Green Spectroscopy and Analytical

Techniques (18)

2009, 42 (6–7)

Trends in Analytical

Chemistry

Green Analytical Chemistry (10) 2010, 29 (7)

Analytical and Bioanalytical

Chemistry

Green Analytical Methods (7) 2012, 404 (3)

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Table 2.

Comparison of analyte-extraction strategies

Liquid-liquid Membrane mediated Solid

Low preconcentration level Medium preconcentration High preconcentration

Single step Simultaneously two steps Two separate steps

Easy automation Relatively easy automation Very easy automation

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